The neocortex of mammals is a highly evolved, differentiated structure. A distinct feature of the neocortex is its subdivision into a number of cortical "areas" that can be distinguished on morphological, connectional and functional bases. Recent evidence indicates that at least some of the features characteristic of areal differentiation of the neocortex are not apparent during the early stages of its development. For example, the distributions of three major classes of cortical projection neurons, pyramidal tract, callosal and corticotectal, are limited in the adult to specific regions, but are found throughout the entire immature neocortex. The developmental restriction of these populations of projection neurons is brought about by a process of selective collateral elimination that can be influenced by various manipulations. These observations support the notion that the neocortex is comprised of repeating units that early on are more-or-less alike, and that areal differential is largely an epigenetic phenomenon. Here I propose a series of experiments designed to test further the idea of cortical homogeneity by determining to what degree heterotopic cortical transplants express features uniquely associated with a specific region of cortex. These experiments will take advantage of the very distinctive cytoarchitectural differentiation of the barrel- field region of the primary somatosensory cortex of the rat. Specifically, we propose to transplant pieces of two dissimilar types of neocortex, visual and motor cortex, taken from late fetal donors to the barrel field of newborn rats, and assay for the development of barrels within the transplants using a number of complementary indicators of morphological differentiation - the acetylcholinesterase and cytochrome oxidase histochemical methods, and Nissl stains. Further with axonal tracing techniques, we will examine the distribution of callosal and thalamic inputs to the transplants to determine if the connectional specificities of the normal barrel field are mimicked in the transplants. Finally, we will make use of the (3H)2- Deoxyglucose method of monitoring neuronal activity to assess the functional organization of the transplants.